Recombinant DNA technology has been used to express proteins in both prokaryotic and eukaryotic cells for a variety of purposes. Typically, a DNA sequence encoding a protein is provided in an expression vector in which it is operably linked to a promoter and the vector is transfected into a host cell to express the protein. The choice of the host cell depends on the particular protein to be expressed and the downstream application of the expressed protein. While using prokaryotic cells such as bacterial cells to produce proteins are often more cost-efficient than using eukaryotic cells, eukaryotic proteins expressed in bacterial cells may not fold properly and lack the posttranslational modifications that occur in eukaryotic cells.
Several unique systems have been developed for producing eukaryotic proteins in eukaryotic cells such that the proteins produced display correct folding as well as important posttranslational modifications to make them biologically active and functional. One example is the Baculovirus vector system for producing eukaryotic proteins in insect cells. In this system, a desired gene is introduced into a nonessential region of Baculovirus genome via homologous recombination with a transfer vector containing the gene in co-transfected cells. The production of foreign protein is then achieved by infection of additional insect cell cultures with the resultant recombinant virus.
Proteins can also be expressed in eukaryotic cells such as insect cells using the traditional expression vector strategy in which a gene of interest is genetically engineered to be under the control of a promoter. This strategy depends on the availability of promoters that can drive transcription in insect cells or other eukaryotic cells. If a relatively large amount of protein is to be produced, a strong promoter is desirable.